Glasgow was attacked because terrorists perceive Scotland to be an "easier target", a terrorism expert has said.

Rohan Gunaratna, author of the book Inside al Qaeda, Global Network of Terror, said there had been no obvious signs the city's airport would be hit.

He added that the incident had nothing to do with Prime Minister Gordon Brown being a Scot.

Mr Gunaratna testified for the US Government in a suspected terror trial in Miami earlier this week.

The former research fellow of St Andrews University's centre for the study of terrorism and political violence said terrorists could strike without warning anywhere in Scotland.

He said: "We have seen a very significant amount of terrorist propaganda during this particular period, but there has been nothing to indicate that there would be a terrorist attack in the UK - nothing specific.

"What is happening now are attempts to retrace and to see if there were any early indicators, or what you call pre-attack indicators."

I think it was about simply looking for targets in an area which is easier to operate in

Rohan Gunaratna

The failed car bombings on Glasgow and London may be followed by other attempted attacks, Mr Gunaratna said.

He added: "Certainly this is a sustained threat, this is a threat we will see more of and will continue for a very long period of time.

"The strategy to fight it is to go after the cells planning, preparing and executing attacks."

He said the government would have to work hard to counter an "ideology of hate" but that terrorist groups, meanwhile, were drawing new followers from across society.

Court case

Mr Gunaratna said: "Terrorists have been able to recruit from the rich, the poor, the educated and the less well educated. The authorities will have to work harder to determine what in fact is taking place."

However, he disputed suggestions the attack on Glasgow was orchestrated to happen in what some have said to be Gordon Brown's "backyard".

He said: "I think it was about simply looking for targets in an area which is easier to operate in."

On Monday, Mr Gunaratna testified for the US government in a court case alleging conspiracy to "murder, kidnap and maim" Russians, Serbs and other ethnic people overseas and providing "material support" for terrorist activity between 1993 and 2001.